2 Answers
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"Purchase the most fuel-efficient vehicles in the class of car they wish to drive"

And that rabbis should encourage congregants to do the same.

But I think "prohibited to drive anything non-hybrid" is a strong word.

While most (all?) of us would agree that consuming less gasoline is better, where would we draw a line at a prohibition? "Prohibited to drive if you could walk? Prohibited to drive a four-seater hybrid if you could afford a more-efficient two-seater hybrid?" Why specifically hybrids? What if solar-power is better?

While we have a religious value here of conservation, it's hard to automatically turn that into a bona-fide prohibition. ("Bal tashchis", "don't waste", for example, does not mean you can never, ever use disposable dishes. There are a few specific prohibitions of bal tashchis, and then a guiding religious principle.)

Now a community certainly has the right to enact a very specific prohibition based on a religious value, and that enactment would be binding in halacha on that community. But communal enactments are rare today, and I don't really see that happening here.

In the meantime, we have a religious "should", not a religious "must"; often "shoulds" conflict with other "shoulds" (do I buy a regular car and give the rest of the money to charity instead?); the choice that best serves G-d for one person in his situation may be different than for someone else.

"'dina d'malchusa dina' ('the law of the kingdom is [torah] law') would most likely apply and make it prohibited Judaism-wise as well. (Though dina d'malchusa is a complex subject.)" As you said, It's a complex subject. Many laws don't fall under that category for various reasons. Just saying.
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MosheSep 2 '10 at 18:45

Am I halachicaly obligated to support the government when the government itself never asked for support?
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Joe ShmoeOct 11 '10 at 21:16

The question would apply also - Is it Assur to use gas to heat your house if you can use solar?
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Gershon GoldOct 11 '10 at 23:53

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Also, this question presumes that the cars being considered are all equal except for fuel-efficiency, which isn't true. If a particular non-hybrid is safer than any hybrid in that class, I'm not sure fuel-efficiency would win if you asked the shaila.
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Monica Cellio♦Oct 7 '11 at 14:19

Why should it be?
The only reason it should be obligated is because of "Veshomarta meod" and global warming will kill us all.

Even with all the evidence for global warming, and assuming changing MY car will make a difference, is there more evidence that changing to a hybrid will solve those problems than the amount of evidence that smoking is fatal in the 60's when Igros Moshe permitted smoking for the lack of evidence that smoking is fatal?